Wildlife conservationist Morad Tahbaz, 66, was returned to custody after being allowed out on furlough last month.

He was temporarily released on the same day charity worker Nazanin Zaghari-Ratcliffe and retired civil engineer Anoosheh Ashoori were freed.

His daughter Roxanne said her family was led to believe that he would be included in any deal negotiated at the time, alongside the two others.

They now feel abandoned by the UK government, she said, as her father, who was born in London, is now back in prison.

Her mother has also been placed under a travel ban by the Iranian authorities, Ms Tahbaz said.

Speaking at her protest outside the Foreign Office on Wednesday morning, she said: "We're here today because it's been one month since Nazanin and Anoosheh have come home, and my father's still sitting in prison and my mother's still on a travel ban.

"So we're hoping to have the press and the media help us to call on the government and on the Foreign Secretary [Liz Truss] to keep her promise and bring him home to us, so we can be reunited as a family."

'It's really about action at this point'

In March, the UK said it had secured Mr Tahbaz's furlough after the government finally agreed to settle a £400 million debt to Iran dating from the1970s.

Two days later Mr Tahbaz was forced to return to Evin prison.

The Foreign Office had said he was moved to a residential location in Tehran but Ms Tahbaz said he was taken back to the prison shortly afterwards.

Asked if she would like the government to apologise after she said it misled her family over her father's fate, she said: "I'd be happy to forgo the apology if they brought my parents home. It's really about action at this point, instead of just words."

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Mr Tahbaz, who also has Iranian citizenship, was arrested during a crackdown on environmental activists in January 2018.

He is a prominent conservationist and board member of the Persian Wildlife Heritage Foundation, which seeks to protect endangered species.

He was sentenced to 10 years in prison with his colleagues on vague charges of spying for the US and undermining Iran's security.

The Foreign Office said: "The Iranian government committed to releasing Morad from prison on an indefinite furlough. Iran has failed to honour that commitment.

"Continuing his horrendous ordeal sends a clear message to the international community that Iran does not honour its commitments. We continue to urge the Iranian authorities at every opportunity to release him immediately."